There are used, as an analgesic, a narcotic analgesic (morphine and the like), a non-narcotic analgesic (aspirin, indomethacin and the like) and a narcotic-antagonistic analgesic (pentazocine and the like). A narcotic analgesic exerts its analgesic effect by inhibiting an algesic excitation transmission mainly in a central system. A non-narcotic analgesic exerts its analgesic effect by inhibiting the production of a dorologenic substance mainly in a peripheral system. A narcotic-antagonistic analgesic exerts its analgesic effect by a mechanism similar to that of a narcotic analgesic.
Nevertheless, there are no useful analgesics against a chronic pain which cannot be suppressed by a morphine, an allodynia associated with herpes zoster or a hyperalgesia, and an excellent analgesic has been still demanded.
Recently, an opioid receptor such as mu (μ), delta (δ) or kappa (κ) opioid receptor was identified, and a further novel subtype receptor, namely, nociceptin receptor (ORL-1) was identified while its intrinsic agonist nociceptin was also found. The nociceptin receptor (ORL-1) agonist is suggested to be effective in treating a neural inflammation, and this agonist is a highly effective analgesic which has less psychological side effects and causes less indulgence.